1. Field of the Art
This invention relates to induction heating and in particular to a method and apparatus for induction heating articles made of sheet metal as they are moved continuously and rapidly past an induction heating coil.
2. Brief Description of the Prior Art
Induction heating metal articles for a variety of purposes such as sealing containers, surface hardening metal parts, brazing, cooking, and curing coatings, among others, is well known. Induction heating involves passing a high frequency alternating current through a copper coil to create magnetic fields or magnetic lines of force in the area surrounding the current carrying conductor coil. When a metal article is placed in the magnetic field, heat is produced in the article as a result of eddy current losses and hysteresis losses (magnetic materials only). The eddy current losses are resistant losses resulting from circulating currents induced in the metal article by virtue of differences in electrical potential at various locations in the metal. This difference in potential is caused by the alternating magnetic fields passing through the metal article.
Heretofore, induction heating of articles made of thin sheet metal has sometimes been unsatisfactory due to non-uniform heating of the articles. It is well known that the eddy current losses which heat an article vary inversely as the square of the distance from the coil conductor. Consequently, various portions of a workpiece will be heated to greatly varying degrees depending on proximity to the coil. Induction heating also produces a so-called "edge effect" phenomenon whereby the edge of a thin sheet of metal will be heated first, generally regardless of its disposition within the magnetic field. Consequently, when a foil disc or can end is positioned in or moved through the magnetic field produced by an induction heating coil, the edge of the article is heated first, and may be overheated, before the center of the article is heated to the desired temperature. When the peripheral edges of a sheet metal article are heated before the center of the article, the edges can be melted or burned off, or the article may be warped due to the non-uniform heating.
Many ideas have been tried for rapidly and uniformly heating an entire sheet metal article, but without much success. For example, sheet metal articles have been rotated as they have been moved past a coil and have been moved through box or stacked coils rather than beneath pancake coils. The patent art is replete with disclosures of methods and apparatus for induction heating of metal articles as is disclosed in U.S. Pat. Nos. 2,479,980, 2,946,168, 3,057,988, 3,548,140, 3,794,802 and Canadian Pat. No. 818,966 among others.
The prior art is lacking in a disclosure of induction heating apparatus and methods in which sheet metal articles can be rapidly heated throughout. The prior art is also lacking in induction heating apparatus and methods for permitting modulation or adjustment of the location of maximum flux intensity of the magnetic field.